The present invention relates to apparatus for bending or shaping glass in sheet form while heated to a plastic state.
Apparatus is described in U.S. Pat. Nos. 3,831,239 and 4,203,754 and British Pat. No. 1,134,775, which are incorporated herein by reference, for continuously bending heated glass sheets in a horizontal position. As described in such patents, such apparatus includes a succession of arched rods, each of which is mounted on a curved rod and is covered with a tubular casing in the form of a thin layer of material. The rods are mounted so that each rod is rotatable about an axis passing through the ends of that rod; and a mechanical linkage couples the rods together so that they may all be rotated simultaneously. Such rotation may extend from a first position in which the plane defined by the curvature of each rod lies in a surface defined by the ends of all the rods to a second position in which the plane of each rod is perpendicular to the surface defined by the ends of the rods. With the rods in this second position, and in any other position between the first and second position, the rods define a transversely curved surface or envelope.
A roller is rotatably mounted on each rod and connected to a suitable drive mechanism so that the rollers move heated glass sheets across them. To permit the roller to rotate about the arched rod, the roller is relatively flexible in the axial direction. Since the angular position of the rollers defines a transverse contour, the heated glass sheets sag under their own weight as they are moved over the rollers and conform themselves to the contour defined by the rods. As described in U.S. Pat. No. 4,203,754, the relative height of the rollers may also vary so as to impart a longitudinal curvature to the glass sheets.
To withstand the glass bending temperature, which ranges from at least 600.degree.-700.degree. C., the rollers must be made of refractory material. In addition, because the rods are curved, the rollers are subjected to considerable mechanical stress as they rotate about the rod. Each segment of their length is alternately being compressed and stretched in a single rotation about the curved portion of the rod. To accommodate such stress, the rollers must have relatively high flexibility in the axial direction. At the same time, to avoid distorting the glass sheet as it is moved across the casing, the rollers must have good torsional resistance. Because of the stress imposed on the rollers, they tend to deteriorate relatively quickly and have to be replaced fairly often at significant expense for labor and materials and equipment downtime.
It is known in the art to make wound metal tubes which consist, for example, of a metal strip with an S section bent in a spiral whose neighboring coils fit into one another. The loose fit of the coils allows the fit sufficient play in the axial direction so as not to create harmful traction or compression stresses if such a tube is used as the roller on an arched rod in glass bending apparatus. However, these tubes do not have sufficient torsional strength for use in this application. Depending on the direction of the tube spiral and the friction that opposes rotation of the tube, the spiral can be caused to increase in diameter or to contract. In the latter case, the spiral will tighten on the rod and perhaps lock thereon. In either case, the wound metal tube can be damaged and, in some circumstances, destroyed.